Power electronics, as an enabling technology, enhances functionality and performance of electrical systems through effective and efficient utilization of electrical energy. The main aims of power converter circuits are to control, convert, and condition electrical power flow from one form to another through the use of solid-state electronics. The power level in these systems ranges from less than one watt in wireless inductive link for biomedical implants to tens, hundreds, or thousands of watts in power supplies for office equipment and to kilowatts or megawatts in power flow controller for electric power transmission.
A power converter circuit generally comprises three main parts: an input filter, a high-frequency switching network, and an output filter. The switching network is the core power processing unit that manipulates power from available source into desired electrical output form for the load, with low power dissipations in the power semiconductor switching devices. The input filter is used to prevent unwanted radiated or conducted noise, generated by the switching network, from getting into the source, and to ensure compliance with regulatory electromagnetic compatibility standards. The output filter is used to pass wanted electrical output form and attenuate unwanted noise to the load. Both the input filter and the output filter can be made using passive components, such as resistors, capacitors, and inductors.
Popular forms of such filters include second-order inductor-capacitor and third-order inductor-capacitor-inductor. These filters provide good attenuation of switching harmonics with small filter components. In principle, the physical size of the filter decreases with an increase in the system order. It is thus an attractive strategy to use high-order filters to increase the system power density, energy efficiency and dynamic response. However, high-order filters exhibit multiple resonant frequencies that would cause unwanted oscillation. A remedial measure to alleviate this problem is to introduce either passive or active damping into the system. Passive damping is simple in structure but introduces system losses; active damping does not introduce additional losses but results in a more complex control system and limited bandwidth.
In practice, switching devices and passive components are not-ideal (i.e., not lossless). Major power losses in the system is usually in the conduction and switching losses of the switching network, and the ohmic and magnetic core losses of the filters.
Although recent advances in new and emerging materials, device technologies, and network topologies have resulted in reducing the losses of switching devices and increasing the operating frequency for reducing the filter size, the filter sections still occupy considerable space and constitute a major part of the total power loss. For example, the total volume occupied by the input and output filter sections in a power factor pre-regulator or static synchronous series compensator could range from 20% to 30% of the total volume. The total power dissipation of power filters can range from 2% to 5% of the power processed by the system.
The ever-increasing density of power converter circuits is straining engineers' and designers' abilities to squeeze space for the filters without sacrificing performance.
In particular, thermal management and electromagnetic couplings among different components within a limited space poses significant challenges to engineers and designers. Yet, there has been no significant enhancement and development in the filter structure and design in today's power converter circuits. Thus, the filter section remains a key limiting factor in advancing the power density and performance of the power converter circuits.
Recently, a new concept for performing input filtering (as a direct substitute for passive filters) in power converter circuits has been proposed. US20150364991A, US20150362933A and US20150364989A show some exemplary systems that incorporate this new concept. Basically, in this concept, a series pass element is used to profile the current drawn from the power supply and an input capacitor is arranged at the input of the power converter circuit to absorb the high-frequency current generated by the power converter circuit. The voltage across the series pass element is regulated by controlling the input voltage of the power converter circuit through adjusting the switching frequency and duty cycle of the power switches in the power converter circuit. In these types of systems, when the input current is small and the voltage of the power supply suddenly increases, the input capacitor cannot be charged up to the required voltage level quickly through the supply current (even if the power converter circuit does not draw any current), and so the input voltage of the power converter circuit is unable to closely follow the sudden change in the voltage of the power supply. As a result, a significant voltage stress (increase) will be applied across the series pass element. Such voltage stress may adversely affect the operation life and performance of the series pass element.